This invention relates generally to gas flow measuring systems and more particularly to pulmonary measurement devices known as spirometers. More particularly, the invention relates to gas flow measuring systems of the open end flow type in which the flow in a channel is caused to pass a restriction in order to develop a pressure difference across the restriction in proportion to flow in the channel. Devices of this type in spirometers are known as pneumotachs. Once a pressure difference has been established, it is possible to measure and integrate the pressure difference to obtain values of flow rate and total volume through the channel or pneumotach.
Reference is made to U.S. Pat. No. 3,924,612 entitled SPIROMETER APPARATUS AND METHOD issued Dec. 9, 1975 to Philip T. Dempster and John Y. Pun. As therein disclosed, a rotary air pressure chopper was employed to pneumatically ground a pressure transducer measuring the pressure difference caused by the restriction in the pneumotach. The output pulses produced were measured against a reference level taken at atmospheric pressure. That system is in general a shunt type system in which the air chopper is synchronized with an electronic circuit which is driven to zero reference level during the atmospheric shunt referencing of the chopper and to develop a signal indicative of the pressure developed across the restriction in the pneumotach during the other portion of the cycle. Suitable electronic circuitry including a sample and hold circuit and buffer amplifiers were there provided for smoothing the sampled output and developing a continuous signal, the value of which is proportional to the flow through the pneumotach. The device disclosed in U.S. Pat. No. 3,924,612 works satisfactorily but possesses several limitations such as the need to use a small restriction and relatively large size smoothing chamber connected in series between the measuring transducer and the pneumotach. In practice, the restriction is subject to clogging, and the large size smoothing chamber required makes sensitive measurements of flow difficult. That system also had a continuous leak of gas from the system due to shunt operation and was not well suited for bi-directional flow measurement.
More specifically, there is a need for a gas flow measuring system which is particularly adapted for bi-directional flow measurement in a flow channel and which does not permit leakage from the channel. Bi-directional measurements are needed in applications, for example, where a patient is connected to a ventilator which is basically a pump for facilitating forced breathing of the patient. In such applications, measurement of the flow both during inhalation and exhalation is desired in a completely closed system and the flow measuring device should be capable of operating independently, i.e., without being affected by the pressure in the flow circuit. In many cases the flow being measured may be quite a bit smaller than the system pressure. In other applications of industrial consequence the pressure in the flow circuit may be relatively high and it may be highly desirable that zero leakage occur from the flow measuring system. There is, therefore, a need for a new and improved flow measuring system which will overcome the above limitations and disadvantages.
The present invention also finds particular application as a gas flow analyzer in carbon monoxide, carbon dioxide, nitrogen and oxygen flow measuring systems and in many other gas flow measuring systems in which a toxic or gaseous flow is to be measured and which must be maintained in isolation from atmosphere. Other applications include neonate, i.e. newborn baby monitoring especially for premature babies under treatment for hyaline membrane disease where a continuous pressure is applied to the patient together with a simulated respiratory pattern. In such systems, gas flow must be measured by systems also capable of ignoring the common mode line pressures as well as capable of bi-directional measurement.
The general method employed in the present invention utilizes classical techniques in which the flow is measured as the electrical output of a pressure transducer which is linearized as the pressure transducer senses the flow across a pneumotach restriction as will be set forth. Volume measurement is provided by time integration of the flow measurement.
As used herein, pneumotach means any device incorporated as part of a flow channel and having a restriction therein for developing a pressure difference on opposite sides A, B thereof in response to flow through said restriction and in said channel and is not restricted to medical diagnostic apparatus.